Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus includes a phase adjuster to adjust a phase of an optical wave traversing an optical element of the phase adjuster during exposure of a pattern on a substrate. In an embodiment, the optical element is a heat controllable optical element in a projection system of the lithographic apparatus. In use, the pattern is illuminated with an illumination mode including an off-axis radiation beam. This beam is diffracted into zeroth-order and first-order diffracted beams oppositely and asymmetrically inclined with respect to an optical axis. An area is identified where the first-order diffracted beam traverses the optical element. An image characteristic of an image of the pattern is optimized by calculating a desired optical phase of the first-order diffracted beam in relation to the optical phase of the zeroth-order diffracted beam. The phase adjuster is controlled to apply the desired optical phase to the first order diffracted beam.

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/476,044, filed Jun. 1, 2009, now allowed, which claimspriority and benefit under 35 U.S.C. §119(e) to U.S. Provisional PatentApplication No. 61/129,080, entitled “Lithographic Apparatus and DeviceManufacturing Method”, filed on Jun. 3, 2008. The content of each of theforegoing applications is incorporated herein in its entirety byreference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used toimpart a beam of radiation with a pattern in its cross-section, thepattern corresponding to a circuit pattern to be formed on an individuallayer of the IC. This pattern can be imaged or transferred onto a targetportion (e.g. comprising part of, one, or several dies) on a substrate(e.g. a silicon wafer). Transfer of the pattern is typically via imagingonto a layer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an image of the entire pattern ontothe target portion at one time, and so-called scanners, in which eachtarget portion is irradiated by scanning the pattern through a radiationbeam in a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

In the semiconductor manufacturing industry there is increasing demandfor ever-smaller features and increased density of features. Thecritical dimensions (CDs) are rapidly decreasing and are becoming veryclose to the theoretical resolution limit of state-of-the-art exposuretools such as steppers and scanners as described above. Conventionaltechnologies aimed at enhancing resolution and minimizing printable CDinclude reducing the wavelength of the exposure radiation, increasingthe numerical aperture (NA) of the projection system of the lithographicapparatus, and/or including features in a patterning device patternsmaller than the resolution limit of the exposure tool so that they willnot print on the substrate, but so that they will produce diffractioneffects which can improve contrast and sharpen fine features.

SUMMARY

Application of such conventional resolution enhancement techniqueshowever may lead to a reduction of depth of focus within which, forexample, imaging of desired patterns at resolution limit can beexecuted. A reduced depth of focus may lead to printed pattern errorsbeyond tolerance when, for example, a residual substrate unflatnesscannot be compensated for during exposure.

It is desirable, for example, to alleviate, at least partially, aproblem associated with a limited range of useable depth of focus usingan embodiment of the invention.

According to an aspect of the invention, there is provided alithographic apparatus comprising an illumination system configured tocondition a radiation beam with an illumination mode including anoff-axis radiation beam emerging from an illumination pole and inclinedat an angle with respect to an optical axis; a support constructed tosupport a patterning device, the patterning device being capable ofimparting the radiation beam with a pattern in its cross-section to forma patterned radiation beam and further being capable of diffracting theoff-axis radiation beam into a zeroth-order diffracted beam and afirst-order diffracted beam oppositely and asymmetrically inclined withrespect to the optical axis; a projection system having a pupil planeand configured to project the patterned radiation beam onto a targetportion of the substrate; a phase adjuster constructed and arranged toadjust a phase of an electric field of a radiation beam traversing anoptical element of the phase adjuster disposed in the pupil plane; and acontroller constructed and arranged to retrieve data representative ofthe pattern and of the illumination mode, to identify an area where thefirst-order diffracted beam traverses the pupil plane, to optimize animage characteristic of an image of the pattern by calculating a desiredoptical phase of the first-order diffracted beam in relation to theoptical phase of the zeroth-order diffracted beam, to map the area on aportion of the optical element, and to apply heat to or extract heatfrom, the portion to change an index of refraction of the portion of theoptical element in accordance with the desired optical phase.

According to an aspect of the invention, there is provided alithographic apparatus comprising an illumination system configured tocondition a radiation beam with a quadrupole illumination mode includinga first and second beam, emerging from a respective first and adjacentsecond pole and both inclined at an angle with respect to an opticalaxis; a support constructed to support a patterning device, thepatterning device being capable of imparting the radiation beam with apattern in its cross-section to form a patterned radiation beam andfurther being capable of diffracting the first beam into a firstzeroth-order beam and a first first-order beam oppositely andsymmetrically inclined with respect to the optical axis, and ofdiffracting the second beam into a second zeroth-order beam and a secondfirst-order beam oppositely and asymmetrically inclined with respect tothe optical axis; a projection system having a pupil plane andconfigured to project the patterned radiation beam onto a target portionof the substrate; a phase adjuster constructed and arranged to adjust aphase of an electric field of a radiation beam traversing an opticalelement of the phase adjuster disposed in the pupil plane; and acontroller constructed and arranged to retrieve data representative forthe pattern and the quadrupole illumination mode, to identify an areawhere the second first-order beam traverses the pupil plane, to optimizedepth of focus of an image of the pattern by calculating a desiredoptical phase of the second first-order beam, to map the area on aportion of the optical element, and to apply heat to or extract heatfrom, the portion to change an index of refraction of the portion of theoptical element in accordance with the desired optical phase.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising transferring a pattern from a patterningdevice onto a substrate, the method including illuminating, with aradiation beam having an illumination mode including an off-axisradiation beam emerging from an illumination pole and inclined at anangle with respect to an optical axis, a patterning device, thepatterning device imparting the radiation beam with a pattern in itscross-section to form a patterned radiation beam and further diffractingthe off-axis radiation beam into a zeroth-order diffracted beam and afirst-order diffracted beam oppositely and asymmetrically inclined withrespect to the optical axis; projecting the patterned radiation beam viaa pupil plane onto a target portion of the substrate; adjusting a phaseof an electric field of a radiation beam traversing an optical elementdisposed in the pupil plane, the adjusting including retrieving datarepresentative of the pattern and of the illumination mode, identifyingan area where the first-order diffracted beam traverses the pupil plane,optimizing an image characteristic of an image of the pattern bycalculating a desired optical phase of the first-order diffracted beamin relation to the optical phase of the zeroth-order diffracted beam,mapping the area on a portion of the optical element, and applying heatto or extracting heat from, the portion to change an index of refractionof the portion of the optical element in accordance with the desiredoptical phase.

According to an aspect of the invention, there is provided a devicemanufacturing method comprising transferring a pattern from a patterningdevice onto a substrate, the method including illuminating, with aradiation beam with a quadrupole illumination mode including a first andsecond beam, emerging from a respective first and adjacent second poleand both inclined at an angle with respect to an optical axis, apatterning device, the patterning device imparting the radiation beamwith a pattern in its cross-section to form a patterned radiation beamand further diffracting the first beam into a first zeroth-order beamand a first first-order beam oppositely and symmetrically inclined withrespect to the optical axis, and diffracting the second beam into asecond zeroth-order beam and a second first-order beam oppositely andasymmetrically inclined with respect to the optical axis; projecting thepatterned radiation beam via a pupil plane onto a target portion of thesubstrate; and adjusting a phase of an electric field of a radiationbeam traversing an optical element disposed in the pupil plane, theadjusting including retrieving data representative for the pattern andthe quadrupole illumination mode, identifying an area where the secondfirst-order beam traverses the pupil plane, optimizing depth of focus ofan image of the pattern by calculating a desired optical phase of thesecond first-order beam, mapping the area on a portion of the opticalelement, and applying heat to or extracting heat from, the portion tochange an index of refraction of the portion of the optical element inaccordance with the desired optical phase.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 illustrates a phase adjuster configured to change a phase of anelectromagnetic wave traversing a projection system of the lithographicapparatus;

FIG. 3 illustrates an optical element included in the phase adjuster;

FIG. 4 depicts temperature controllable portions of the optical elementincluded in the phase adjuster;

FIG. 5 a quadrupole illumination mode;

FIG. 6 illustrates a pattern generated by a patterning device;

FIG. 7 depicts diffracted beams originating from a single pole of thequadrupole illumination mode;

FIG. 8 depicts diffracted beams originating from a pole of thequadrupole illumination mode adjacent to the pole of FIG. 7;

FIG. 9 depicts areas traversed by diffracted beams to which a change ofphase is applied in an embodiment;

FIG. 10 a illustrates a first aspect of exposure latitude versus depthof focus for printing the pattern of FIG. 6 in the presence of phasechanges applied according to an embodiment;

FIG. 10 b illustrates a second aspect of exposure latitude versus depthof focus for printing the pattern of FIG. 6 in the presence of phasechanges applied according to an embodiment;

FIG. 10 c illustrates the first aspect of exposure latitude versus depthof focus for printing the pattern of FIG. 6 in the absence of phasechanges; and

FIG. 10 d illustrates the second aspect of exposure latitude versusdepth of focus for printing the pattern of FIG. 6 in the absence ofphase changes.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus 100 according toan embodiment of the invention. The apparatus 100 comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation, for example generated by an excimerlaser operating at a wavelength of 248 nm or 193 nm, or EUV radiation,for example generated by a laser-fired plasma source operating at about13.6 nm wavelength);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device. It holds thepatterning device in a manner that depends on the orientation of thepatterning device, the design of the lithographic apparatus, and otherconditions, such as for example whether or not the patterning device isheld in a vacuum environment. The support structure MT can usemechanical, vacuum, electrostatic or other clamping techniques to holdthe patterning device. The support structure MT may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure MT may ensure that the patterning device is at adesired position, for example with respect to the projection system. Anyuse of the terms “reticle” or “mask” herein may be considered synonymouswith the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus 100 is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus 100 may be of a type having two (dual stage)or more substrate tables (and/or two or more patterning device tables).In such “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus 100 may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the patterning device table MT may berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the patterning device table MT may be connected toa short-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus 100 could be used in at least one of thefollowing modes:

In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

In another mode, the support structure MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

The optical arrangement of the apparatus of FIG. 1 uses Koehlerillumination. With Koehler illumination, a pupil plane PP_(i) in theillumination system IL is conjugate to a pupil plane PP_(p) of theprojection system PS. The pupil plane PP_(p) is a Fourier transformplane of the object plane in which the patterning device MA is located.As is conventional, an illumination mode of this apparatus can bedescribed by reference to the distribution of intensity of the radiationof the beam B in the pupil plane PP_(i) of the illumination system. Thedistribution of intensity in the pupil plane PP_(p) of the projectionsystem PS will be substantially the same as the distribution ofintensity in the pupil plane PP_(i) of the illumination system, subjectto diffraction effects of the pattern of the patterning device MA.

The projection system PS comprises a phase adjuster 110 constructed andarranged to adjust a phase of an electric field of an optical radiationbeam traversing the projection system. As schematically shown in FIG. 2,the phase adjuster 110 may comprise an optical element 310 of a materialsubstantially transmissive for radiation of the beam B. In anembodiment, the optical element 310 may be reflective for radiation ofthe beam 340. The phase adjuster 110 may further comprise a controller340. An optical path length for a wave traversing the element 310 isadjustable in response to a signal supplied by controller 340. Theoptical element 310 may be disposed or disposable, for example,substantially in a Fourier Transform plane such as the pupil PPp, andsuch that—in use—it is traversed by diffracted beams DB emanating fromthe patterning device.

FIG. 3 illustrates the phase adjuster 110 in more detail, and shows atop view, along the Z-axis, of the optical element 310. An adjustment ofa phase of an optical wave traversing the element 310 may be obtained byapplying heat to, or removing heat from, a portion 320 of the opticalelement 310, thereby introducing a local change of index of refractionof the material of the element relative to the refractive index of thematerial adjacent to the portion 320. The application of heat can berealized by, for example, transmitting an electrical current through awire 330 having Ohmic resistance and being arranged in contact with theportion 320 of the element and with the controller 340 arranged toprovide the current to the wire 330.

A plurality of adjacent portions of the optical element may be providedwith a corresponding plurality of wires for heating any portionindependently from any other portion. For example, as schematicallyillustrated in FIG. 4, adjacent portions 320-1 up to 320-44 are disposedin adjacent rows and numbered from left to right and from top to bottom.Each portion 320 of the portions 320-1 up to 320-44 is provided withcorrespondingly numbered heating wires 330-1 up to 330-44 (although FIG.4, merely for clarity sake, illustrates this only for the portions 320-4and 320-37). The controller 340 is constructed and arranged such thateach wire can be current-activated independently. This enablesapplication of a spatial phase distribution to an optical wavetraversing the element 310, in accordance with a spatial distribution ofthe temperature over the element 310 in the X,Y plane.

In addition or alternatively, the optical element 310 may include achannel arranged to contain a cooling fluid. The phase adjuster 110 mayinclude a cooling fluid supply and retrieval system connected to thechannel and arranged to circulate cooling fluid at a controlledtemperature through the channel. Like the wires 330, a cooling channelmay be associated with each portion 320; however, alternatively a singlecooling channel may be arranged for all portions 320. A cooling of theelement 310 in combination with heating a portion 320 of the element 310may enable adjusting the temperature of the portion 320 within a rangeof temperatures extending both below and above a nominal temperature.The nominal temperature may, for example, be a specified desiredoperating temperature of the apparatus 100 or of the material of theoptical elements of the projection system PS.

Embodiments of a phase adjuster 110 can be gleaned from U.S. Pat. No.7,525,640. A total number of portions 320 is not limited to 44. Insteadit may in general depend on a desired spatial resolution of temperaturedistributions. For example, a ratio of the area of each of the portions320 to the size of a clear area in the pupil plane PPi of the projectionsystem PS may be between 100 and 1000.

It is noted that the invention is not limited to the specificembodiment, described above, of the phase adjuster 110. Such anembodiment is presented herein for illustrative purposes only.Additional embodiments will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. For example, analternative phase adjuster 110 may include an infrared laser arranged toselectively heat portions 320 of an optical element 310 disposed nearthe lens pupil PP_(p). The infrared radiation may be guided to selectedportions of the optical element by means of, for example, one or morehollow optical fibers. Details of the infrared laser arrangement forthis embodiment can be gleaned from Japanese patent applicationpublication no. JP 2007317847A. In the absence of a cooling arrangement,temperatures of different portions 320 can be arranged to mutuallydiffer from each other by supplying corresponding mutually differentamounts of infrared radiation energy to the corresponding differentportions. A nominal temperature may then be specified as, for example,the average temperature value of the mutually different temperatures.

In the embodiment, a patterning device pattern provided by thepatterning device MA is illuminated using a conventional quadrupoleillumination mode, illustrated in FIG. 5. The beam B includes a firstradiation beam B1 inclined at an angle α1 with respect to the Z-axis(which is substantially normal to the patterning device MA) in a firstplane of incidence PI1 and a second radiation beam B2, also inclined atan angle α1 with respect to the patterning device in a second plane ofincidence PI2. The second plane of incidence PI2 is arranged at an angleφ with respect to the plane PI1. The beam B further includes beams B1′and B2′ respectively arranged symmetrically opposite to the respectivebeams B1 and B2. The angle φ is 90 degrees in the embodiment, but is notlimited to this value. The planes PI1 and PI2 are respectively an X-Zplane and an Y-Z plane in the embodiment, but can also be chosen atother rotational orientations with respect to the Z-axis.

The patterning device pattern 600 may consist of a brick-wall type DRAMpattern as shown in FIG. 6. The axes of the line segments 610 (thelonger axes of the bricks) are substantially parallel to theY-direction. The line segments 610 are arranged at a pitch Px along theX-direction (perpendicular to the longer axes of the bricks). The gaps620 are defined by the separation between two adjacent line segmentsalong a single line. The arrangement of gaps 620 defines a pitch Pyalong the Y-direction.

In the embodiment, the value of pitch Px is 100 nm, and the width of theline segments 610 is 50 nm. The width of the gaps 620 is 65 nm. Theprojection process for printing the pattern 600 is characterized by theprojection system PS having a NA of 1.2 and the radiation of the beam Bhaving a wavelength of 193 nm.

As illustrated in FIG. 7, the patterning device pattern 600 diffractsthe beam B1 into a zero and a first order diffracted beam, respectivelythe beams DB10 and DB11. The beams DB10 and DB11 traverse a FourierTransform plane in the projection system PS, e.g. the pupil plane PPp.The angle α1 is arranged such that the traversing of the pupil plane PPpoccurs in respective opposite areas AE10 and AE11 disposed symmetricallywith respect to the optical axis OA (parallel to the Z-axis), atrespective equal distances d10 and d11 (d10=d11) from the optical axisOA.

As illustrated in FIG. 8, the patterning device pattern furtherdiffracts the beam B2 into a zero and a first order diffracted beam,respectively beams DB20 and DB21. The beams DB20 and DB21 traverse aFourier Transform plane in the projection system PS, e.g. the pupilplane PPp, in respective opposite areas AE20 and AE21. In contrast tothe areas AE10 and AE11, the areas AE20 and A21 are disposedasymmetrically with respect to the optical axis OA, at respectivedifferent distances d20 and d21 from the optical axis OA. The differencebetween the diffracted beams originating from B1 and B2 is related tothe difference between the pitches Px and Py of the line segments 610and the gaps 620. In particular, in comparison to features arranged atshorter pitch, features arranged at a longer pitch are less effective indiffracting radiation away from a zeroth order diffracted beam directionof a patterning device-illuminating beam.

In the presence of quadrupole illumination such as illustrated in FIG.5, an image of a line segment 610 and of a gap 620 can be represented asa sum of 4 images, respectively generated by radiation emanating fromthe four respective poles of the quadrupole illumination intensitydistribution in the illumination pupil plane PPi. A first image im1 isgenerated by the recombination of the beams DB10 and DB11 diffractedfrom B1, see FIG. 7, just above and at the substrate W. This imagecontributes mainly to contrast and resolution along the X-direction inthe image of the brick-pattern 600, resolving the line shaped spacesbetween the bricks 610 along the X-direction better than the gaps 620between the bricks 610. Similarly, diffracted beams generated bydiffraction of the beam BF provide a similar first additional imageim1′. A second image im2 is generated by the recombination of the beamsDB20 and DB21 diffracted from B2, see FIG. 8, just above and at thesubstrate. This second image contributes mainly to contrast andresolution along the Y-direction in the image of the pattern 600,resolving the gaps 620 between the bricks 610 better than the lineshaped spaces between the bricks 610 along the X-direction. A similar,second additional image im2′ originates from diffracted beams generatedby diffraction of the beam B2′ at the patterning device pattern 600.

Near and at the image plane an inclination with respect to the Z-axis ofthe beams DB10 and DB11 is proportional to the distance d10 and d11,respectively, in the pupil plane PPp. Hence, near the image plane, thebeams DB10 and DB11 recombine at opposed and substantially equalinclinations with respect to Z-axis (which is substantially normal tothe substrate W) to form the first image. The type of imaging where twobeams recombine at equal but opposite angles of inclination is referredto as symmetric two-beam imaging. The formation of the first additionalimage im1′ is characterized by such a symmetric two-beam imaging aswell.

With the combined two images im1 and im1′ there is associated a depth offocus, denoted here by DoFx. It is a distance along the Z-axis withinwhich a defocus of the substrate W during exposure is not causing anerror beyond tolerance of the width of the line segments 610 as printedin the resist. This width is the critical dimension CD, the smallestprintable dimension of the pattern. The depth of focus DoFx is 300 nm inthe embodiment. Similarly, one can associate a depth of focus with theimaging of the gaps 620, i.e., with the combined images im2 and im2′ Incontrast to the imaging associated with the radiation originating fromthe beams B1 and B1′, the image formation of image im2 by the beams DB20and DB21 (originating from the beam B2) is not a symmetric two-beamimaging, because the angles of inclination of the beams DB20 and DB21with respect to the Z-axis, near the image plane, are different. Such anon-symmetric two-beam imaging is less desirable because the lack ofsymmetry may lead to a reduction of depth of focus. In the embodiment aconsequence would be that the depth of focus of the combined images im1and im1′ is larger than the depth of focus of the combined images im2and im2′. The latter depth of focus is referred to as DoFy. In theembodiment, to substantially avoid such a difference between depths offocus DoFx and DoFy, a first change of optical phase is applied to thebeam DB21 and a second change of optical phase is applied to the beamDB21′; these changes of phase are changes relative to the phase of therespective zeroth order beams DB20 and DB20′. Further, no such relativephase changes are applied to the other zeroth order and first orderdiffracted beams DB10, DB11, DB10′, and DB11′.

The phase adjuster 110 is used to provide desired phase changes to thebeams DB21 and DB21′. First, data representative for the patterningdevice pattern 600 and the illumination mode are retrieved, bycontroller 340, from a patterning-device data-file and a file includingillumination mode setting data. Next, an intensity distribution in thepupil plane PPp of the projection system is predicted based on dataincluding the pattern and illumination data. Areas AE21 and AE21′ (thelatter not shown in FIG. 8) are identified where the beams DB21 andDB21′ traverse the optical element 310 of phase adjuster 110. Alithographic process optimization, e.g. arranged to optimize depth offocus, is executed by the controller 340. Optimization variables used inthe optimization include the aforementioned first and second change ofoptical phase. Desired first and second optical phases are calculatedand stored. The identified areas AE21 and AE21′ are mapped on theportions 320 of the optical element 310, and portions 320 substantiallytraversed by the respective beams DB21 and DB21′ are identified andtheir addresses in relation to corresponding heating wires and/orcooling channels are stored. In an embodiment, the area AE21 is assumedto correspond to the portions 320-12 and 320-19, as illustrated in FIG.9. Similarly, the area AE21′ is assumed to correspond to the portions320-26 and 320-33.

In the embodiment, the desired phase change for beam DB21 is a fraction90/193 of 2π, and the desired phase change for beam DB21′ is of equalmagnitude but of opposite sign compared to the first phase change. Thecontroller converts the desired phase changes into a respective desiredfirst temperature for the portions 320-12 and 320-19 and a desiredsecond temperature for the portions 320-26 and 320-33, at oppositetemperature intervals from a desired nominal temperature for any of theother portions 320-1 up to 320-44, and determines and appliescorresponding currents to the heating wires (and/or cooling fluidtemperature to the channels).

As noted above, the invention is not limited to the specific embodimentof the phase adjuster 110. For example, an alternative phase adjuster110 may include an infrared laser arranged to selectively heat portions320 of an optical element 310 disposed near the lens pupil PP_(p).

A simulation predicts that applying the first and second phase changesas described above may result in an increase of depth of focus for theimages im2 and im2′.

FIG. 10 a illustrates the simulated exposure latitude versus depth offocus DoFx for printing the widths of the line segments 610 in theX-direction, in the presence of the applied first and second phasechanges. Along the vertical axis of FIG. 10 a, the exposure latitude inpercentage is plotted; along the horizontal axis the depth of focus inmicrometers (μm) is plotted.

FIG. 10 b illustrates the simulated exposure latitude versus depth offocus DoFy for printing the gaps 620 between the line segments 610, inthe presence of the applied first and second phase changes. Along thevertical axis in FIG. 10 b, the exposure latitude in percentage isplotted; along the horizontal axis the depth of focus in micrometers(μm) is plotted.

FIG. 10 c illustrates the simulated exposure latitude versus depth offocus DoFx for printing the widths of the line segments 610 in theX-direction, in the absence of applying first and second phase changes.Along the vertical axis in FIG. 10 c, the exposure latitude inpercentage is plotted; along the horizontal axis the depth of focus inmicrometers (μm) is plotted.

FIG. 10 d illustrates the simulated exposure latitude versus depth offocus DoFy for printing the gaps 620 between the line segments 610, inthe absence of applying the applied first and second phase changes.Along the vertical axis in FIG. 10 d, the exposure latitude inpercentage is plotted; along the horizontal axis the depth of focus inmicrometer (μm) is plotted.

As illustrated in these FIGS. 10 a-d, at an exposure latitude of 1%, thedepth of focus DoFy for an image of a gap increased from approximately125 nm to 180 nm by applying the first and second phase changes. Theincrease of depth of focus may be obtained without substantiallyaffecting overlay performance, that is, without an associated shift ofthe location of an image of a gap 620 in the X-Y plane at the substrateW. In addition, the increase of depth of focus DoFy may be obtainedwithout substantially reducing the depth of focus DoFx associated withcontrast along the X-direction in the image of pattern 600, needed toprint edges of the line segments 610 along the long axis of the bricks610 at a position within tolerance.

In an embodiment, there is provided a lithographic apparatus comprising:an illumination system configured to condition a radiation beam with anillumination mode including an off-axis radiation beam emerging from anillumination pole and inclined at an angle with respect to an opticalaxis; a support constructed to support a patterning device, thepatterning device being capable of imparting the radiation beam with apattern in its cross-section to form a patterned radiation beam andfurther being capable of diffracting the off-axis radiation beam into azeroth-order diffracted beam and a first-order diffracted beamoppositely and asymmetrically inclined with respect to the optical axis;a projection system having a pupil plane and configured to project thepatterned radiation beam onto a target portion of the substrate; a phaseadjuster constructed and arranged to adjust a phase of an electric fieldof a radiation beam traversing an optical element of the phase adjusterdisposed in the pupil plane; and a controller constructed and arrangedto retrieve data representative of the pattern and of the illuminationmode, to identify an area where the first-order diffracted beamtraverses the pupil plane, to optimize an image characteristic of animage of the pattern by calculating a desired optical phase of thefirst-order diffracted beam in relation to the optical phase of thezeroth-order diffracted beam, to map the area on a portion of theoptical element, and to apply heat to or extract heat from, the portionto change an index of refraction of the portion of the optical elementin accordance with the desired optical phase.

In an embodiment, the illumination mode is a quadrupole illuminationmode including a first and a second beam, emerging from a respectivefirst and an adjacent second pole and both inclined at the angle withrespect to the optical axis, the off-axis radiation beam is the secondbeam, and the patterning device is capable of diffracting the first beaminto a zeroth-order beam and a first-order beam oppositely andsymmetrically inclined with respect to the optical axis. In anembodiment, the image characteristic is a depth of focus.

In an embodiment, there is provided a device manufacturing methodcomprising transferring a pattern from a patterning device onto asubstrate, the method including: illuminating, with a radiation beamhaving an illumination mode including an off-axis radiation beamemerging from an illumination pole and inclined at an angle with respectto an optical axis, a patterning device, the patterning device impartingthe radiation beam with a pattern in its cross-section to form apatterned radiation beam and further diffracting the off-axis radiationbeam into a zeroth-order diffracted beam and a first-order diffractedbeam oppositely and asymmetrically inclined with respect to the opticalaxis; projecting the patterned radiation beam via a pupil plane onto atarget portion of the substrate; adjusting a phase of an electric fieldof a radiation beam traversing an optical element disposed in the pupilplane, the adjusting including: retrieving data representative of thepattern and of the illumination mode, identifying an area where thefirst-order diffracted beam traverses the pupil plane, optimizing animage characteristic of an image of the pattern by calculating a desiredoptical phase of the first-order diffi acted beam in relation to theoptical phase of the zeroth-order diffracted beam, mapping the area on aportion of the optical element, and applying heat to or extracting heatfrom, the portion to change an index of refraction of the portion of theoptical element in accordance with the desired optical phase.

In an embodiment, the illumination mode is a quadrupole illuminationmode including a first and a second beam, emerging from a respectivefirst and an adjacent second pole and both inclined at the angle withrespect to the optical axis, the off-axis radiation beam is the secondbeam, and the patterning device diffracts the first beam into azeroth-order beam and a first-order beam oppositely and symmetricallyinclined with respect to the optical axis. In an embodiment, the imagecharacteristic is a depth of focus.

In an embodiment, there is provided a data storage medium having acomputer program stored therein, the computer program containing one ormore sequences of machine-readable instructions configured to execute amethod to adjust a phase of an electric field of a radiation beamtraversing an optical element disposed in a pupil plane, the adjustingincluding: retrieving data representative of a pattern of a patterningdevice to be transferred from the patterning device onto a substrate;retrieving data representative of an illumination mode including anoff-axis radiation beam emerging from an illumination pole and inclinedat an angle with respect to an optical axis, wherein a radiation beamhaving the illumination mode illuminates the patterning device, whereinthe patterning device imparts the radiation beam with a pattern in itscross-section to form a patterned radiation beam and wherein thepatterning device diffracts the off-axis radiation beam into azeroth-order diffracted beam and a first-order diffracted beamoppositely and asymmetrically inclined with respect to the optical axis;identifying an area where the first-order diffracted beam traverses thepupil plane; optimizing an image characteristic of an image of thepattern by calculating a desired optical phase of the first-orderdiffracted beam in relation to the optical phase of the zeroth-orderdiffracted beam; mapping the area on a portion of the optical element;and causing application of heat to or extraction of heat from, theportion to change an index of refraction of the portion of the opticalelement in accordance with the desired optical phase, wherein thepatterned radiation beam is projected via the pupil plane onto a targetportion of the substrate.

In an embodiment, the illumination mode is a quadrupole illuminationmode including a first and a second beam, emerging from a respectivefirst and an adjacent second pole and both inclined at the angle withrespect to the optical axis, the off-axis radiation beam is the secondbeam, and the patterning device diffracts the first beam into azeroth-order beam and a first-order beam oppositely and symmetricallyinclined with respect to the optical axis. In an embodiment, the imagecharacteristic is a depth of focus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1-12. (canceled)
 13. A lithographic apparatus comprising: a projectionsystem configured to project the patterned radiation beam onto a targetportion of the substrate, the patterned radiation beam having azeroth-order beam and an associated first-order beam to form an image ofat least part of a pattern of the patterned radiation beam; a phaseadjuster constructed and arranged to adjust an optical phase ofradiation of the patterned radiation beam traversing the phase adjuster;and a controller constructed and arranged to control the phase adjusterto selectively apply an optical phase change to the first-order beamrelative to the zeroth-order beam.
 14. The lithographic apparatus ofclaim 13, comprising an illumination system configured to condition aradiation beam with an illumination mode including an off-axis radiationbeam emerging from an illumination pole and inclined at an angle withrespect to an optical axis.
 15. The lithographic apparatus of claim 14,wherein: the illumination mode is a multi-pole illumination modeincluding a first and a second beam, emerging from a respective firstand an adjacent second pole and both inclined at an angle with respectto the optical axis; the second beam is used to form the zeroth-orderbeam and the associated first-order beam; and the patterned radiationbeam further comprises a zeroth-order beam and an associated first-orderbeam formed from the first beam, the zeroth-order beam and theassociated first-order beam of the first beam being oppositely andsymmetrically inclined with respect to the optical axis.
 16. Thelithographic apparatus of claim 13, wherein the controller is configuredto cause the phase adjuster to apply heat to or extract heat from anoptical element of the phase adjuster to change an index of refractionthereof in accordance with the optical phase change.
 17. Thelithographic apparatus of claim 13, wherein the controller is configuredto calculate a value of the optical phase change to cause a change in animage characteristic of an image of the pattern.
 18. The lithographicapparatus of claim 17, wherein the image characteristic is a depth offocus.
 19. The lithographic apparatus of claim 13, wherein, in use, thezeroth-order beam and the associated first-order beam are oppositely andasymmetrically inclined with respect to an optical axis of the patternedradiation beam.
 20. A lithographic apparatus comprising: a supportconstructed to support a patterning device, the patterning deviceconfigured to impart a radiation beam with a pattern in itscross-section to form a patterned radiation beam having a zeroth-orderbeam and an associated first-order beam, the first-order beam inclinedat a different angle to an optical axis of the radiation beam than thezeroth-order beam; a projection system configured to project thepatterned radiation beam onto a target portion of the substrate; a phaseadjuster constructed and arranged to adjust an optical phase ofradiation traversing an optical element of the phase adjuster; and acontroller constructed and arranged to identify an area where thefirst-order beam traverses, in use, the optical element and to vary aproperty of the optical element in the area in order to apply an opticalphase change to the first-order beam relative to the zeroth-order beam.21. The lithographic apparatus of claim 20, comprising an illuminationsystem configured to condition a radiation beam with an illuminationmode including an off-axis radiation beam emerging from an illuminationpole and inclined at an angle with respect to an optical axis.
 22. Thelithographic apparatus of claim 21, wherein: the illumination mode is amulti-pole illumination mode including a first and a second beam,emerging from a respective first and an adjacent second pole and bothinclined at an angle with respect to the optical axis; the second beamis used to form the zeroth-order beam and the associated first-orderbeam; and the patterned radiation beam further comprises a zeroth-orderbeam and an associated first-order beam formed from the first beam, thezeroth-order beam and the associated first-order beam of the first beamoppositely and symmetrically inclined with respect to the optical axis.23. The lithographic apparatus of claim 20, wherein the property of theoptical element is index of refraction of the optical element and thecontroller is configured to cause the phase adjuster to apply heat to orextract heat from the optical element to change the index of refractionof the area of the optical element in accordance with the optical phasechange.
 24. The lithographic apparatus of claim 20, wherein thecontroller is configured to calculate a value of the optical phasechange to cause a change in an image characteristic of an image of thepattern.
 25. The lithographic apparatus of claim 24, wherein the imagecharacteristic is a depth of focus.
 26. A device manufacturing methodcomprising transferring a pattern from a patterning device onto asubstrate, the method including: projecting a patterned radiation beamonto a target portion of the substrate, the patterned radiation beamhaving a zeroth-order beam and an associated first-order beam to form animage of at least part of a pattern of the patterned radiation beam;adjusting an optical phase of radiation of the patterned radiation beamusing a phase adjuster, the adjusting comprising controlling the phaseadjuster to selectively apply an optical phase change to the first-orderbeam relative to the zeroth-order beam traversing the phase adjuster.27. The method of claim 26, comprising providing a radiation beam withan illumination mode including an off-axis radiation beam emerging froman illumination pole and inclined at an angle with respect to an opticalaxis.
 28. The method of claim 27, wherein: the illumination mode is amulti-pole illumination mode including a first and a second beam,emerging from a respective first and an adjacent second pole and bothinclined at an angle with respect to the optical axis; the second beamis used to form the zeroth-order beam and the associated first-orderbeam; and the patterned radiation beam further comprises a zeroth-orderbeam and an associated first-order beam formed from the first beam, thezeroth-order beam and the associated first-order beam of the first beamoppositely and symmetrically inclined with respect to the optical axis.29. The method of claim 26, wherein adjusting the optical phase ofradiation comprises causing the phase adjuster to apply heat to orextract heat from an optical element of the phase adjuster to change anindex of refraction thereof in accordance with the optical phase change.30. The method of claim 26, comprising calculating a value of theoptical phase change to cause a change in an image characteristic of animage of the pattern.
 31. The method of claim 30, wherein the imagecharacteristic is a depth of focus.
 32. The method of claim 26, whereinthe zeroth-order beam and the associated first-order beam are oppositelyand asymmetrically inclined with respect to an optical axis of thepatterned radiation beam.